In the production of chemical wood pulps, the wood is subjected to chemical action to dissolve the lignin and leave the cellulose to provide the cellulose fibre that is subsequently used to produce paper and the like. Black liquor is produced and contains the dissolved solids such as lignin and many of the transformed inorganic materials that are used to cook the wood. This black liquor is normally evaporated to remove water and increase its concentration and then is burned in the recovery boiler which functions both to generate heat, which is normally recovered in the form of steam and to recover chemical for reuse in the pulping operation.
Generally, in the kraft system, a smelt is formed and extracted from the bottom of the recovery furnace and the smelt is then dissolved to form what is known as green liquor. The green liquor is treated with lime to convert the green liquor to white liquor, i.e. containing sodium hydroxide and the calcium carbonate produced by this reaction is obtained as a precipitate or sludge which is washed and returned to a lime kiln wherein it is fired to produce lime for reuse in the process.
In many mills, the bottleneck to increasing production is the size of the recovery furnace. Thus, when the capacity of the recovery furnace is exceeded other means to process the black liquor have been sought. For example, it has been suggested to burn the excess liquor in a fluidized bed and produce a particulate product that may later be further processed, for example, by injection onto the reducing bed of the recovery furnace or otherwise reduced to form the source for green liquor.
Swedish patent 453,408 published Feb. 1, 1988 inventor Chaudhrui teaches a concept to recovering of heat values from lignin by precipitating the lignin from excess black liquor and burning it in a lime kiln to aid in the conversion of the sludge to the lime to provide at least a portion of the heat necessary to produce lime. One of the major problems with this technique and the other techniques of which Applicant is aware for handling suspensions of lignin for burning or other purposes is the instability of the lignin in a liquid or semi-liquid suspension as the pumpability of the lignin suspension becomes impaired, making the process difficult, and in many cases, simply not satisfactory.